[PATCH 6.18.y] fuse: avoid 0x10 fault in fuse_readahead when max_pages == 0

[Vlad Poenaru <vlad.wing@gmail.com>]

When fc->max_read is smaller than PAGE_SIZE (common on aarch64 with
64K base pages if the FUSE server advertises a small max_read in INIT),
max_pages = min(fc->max_pages, fc->max_read / PAGE_SIZE) is 0, so
cur_pages is 0 on every outer iteration.

fuse_io_alloc(NULL, 0) then calls fuse_folios_alloc(0, ...), which
calls kzalloc(0, ...) and gets back ZERO_SIZE_PTR == (void *)16.
The "if (!ia->ap.folios)" guard in fuse_io_alloc does not catch
ZERO_SIZE_PTR, so fuse_io_alloc happily returns an ia whose
ap.folios is 0x10.

The inner "while (pages < cur_pages)" loop runs zero times, then
fuse_send_readpages(ia, ...) dereferences ap->folios[0] in
folio_pos(), faulting at virtual address 0x10:

  Unable to handle kernel NULL pointer dereference at virtual address
  0000000000000010
   fuse_readahead+0x14c/0x490
   read_pages+0x80/0x318
   page_cache_ra_unbounded+0x1c0/0x2b0
   page_cache_ra_order+0xb8/0x368
   page_cache_sync_ra+0x210/0x320
   filemap_get_pages+0x290/0xdb0
   generic_file_read_iter+0xd0/0x540
   fuse_file_read_iter+0x8c/0x158
   __arm64_sys_read+0x1a0/0x488

addr2line on the aarch64 vmlinux maps fuse_readahead+0x14c to
fs/fuse/file.c:897 inlined into :999, i.e. "folio_pos(ap->folios[0])"
inside fuse_send_readpages.  The faulting instruction "ldr x8, [x8]"
loads ap->folios[0]; ap->folios was previously loaded as 0x10
(ZERO_SIZE_PTR).

Without this fix the function would also spin forever, since
"nr_pages -= pages" makes no progress when pages stays 0; in practice
the NULL deref masks the spin.

Bail out of the outer loop if cur_pages is 0 -- there is no work we
can issue via FUSE in this iteration, and remaining folios will be
handled by read_pages() falling back to ->read_folio.

Note: this code was rewritten in mainline by commit 4ea907108a5c
("fuse: use iomap for readahead"), which switched fuse_readahead to
iomap and removed the buggy loop entirely.  This patch therefore
applies only to stable branches that still carry the pre-iomap
readahead path.

Fixes: 3eab9d7bc2f4 ("fuse: convert readahead to use folios")
Reported-by: Breno Leitao <leitao@debian.org>
Cc: stable@vger.kernel.org
Signed-off-by: Vlad Poenaru <vlad.wing@gmail.com>
---
 fs/fuse/file.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 6014d588845c..782178124512 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -974,6 +974,16 @@ static void fuse_readahead(struct readahead_control *rac)
 		unsigned cur_pages = min(max_pages, nr_pages);
 		unsigned int pages = 0;
 
+		/*
+		 * If max_pages == 0 (e.g. fc->max_read < PAGE_SIZE on a
+		 * 64K-page kernel), cur_pages is 0 and we cannot make
+		 * progress.  Bailing here avoids passing 0 to fuse_io_alloc,
+		 * which would return an ia whose ap.folios is ZERO_SIZE_PTR
+		 * (0x10) -- later dereferenced by fuse_send_readpages.
+		 */
+		if (!cur_pages)
+			break;
+
 		if (fc->num_background >= fc->congestion_threshold &&
 		    rac->ra->async_size >= readahead_count(rac))
 			/*
-- 
2.53.0-Meta